The present invention relates to overvoltage protection in integrated circuits. More specifically, the present invention provides techniques for protecting integrated circuits from overvoltage conditions of relatively long duration with no significant loss of supply voltage.
Every integrated circuit (IC) is fabricated according to a process with a rated absolute maximum voltage. If any of the pins on an IC are subjected to a potential greater than this rated absolute maximum voltage, the device will be destroyed. Thus, the system designer must ensure that the IC is never subjected to a potential greater than the absolute maximum voltage in a given application. However there are situations where the overvoltage problem is unavoidable and some form of protection circuitry is needed in order to protect the IC.
One current solution involves the use of a zener diode tied between the IC's power pins and ground. When the system supply voltage (VIN) is less than the zener voltage, the zener diode remains off and current flows from VIN to the IC's supply voltage (VCC). The zener diode breaks down and conducts whenever VIN exceeds the set voltage for the particular zener diode. When the zener diode is conducting, excess current flows from VIN through the zener diode to ground, thereby holding VCC at the zener voltage.
The main problem with this solution is that the zener diode must pass large amounts of current and dissipate large amounts of heat. As a result, this solution is only acceptable if the over voltage condition is for a short period (e.g., less than 1 or 2 seconds). Otherwise the zener diode overheats, self destructs, and the IC loses its protection.
A voltage regulator is another potential solution for the overvoltage problem. Common voltage regulators such as the LM7812 can be used to limit the supply voltage at the IC. The main problem to this solution is “dropout voltage,” i.e., the minimum difference between the regulator input voltage and the output voltage. The dropout voltage for a device such as the LM7812 is 1.5–2V. In an application such as automotive audio amplifiers, the supply should deliver the maximum available voltage up to, but not above, the rated IC voltage. Any dropout below the rated voltage is undesirable. So if the input is 14V and the rated voltage is 14V, then the regulator should deliver 14V. However, an LM7812, because of its dropout voltage, can only deliver 12.5V under these circumstances.
An additional problem with the voltage regulator solution is that the maximum output current may not be high enough to satisfy the system needs. For example, an LM340T-12 can source up to about 2.5 A, which only supplies about 30 W at 12V. This is an insufficient power level for applications such as an automotive audio amplifier.
It is therefore desirable to provide overvoltage protection for integrated circuits using techniques which can handle overvoltage conditions having relatively long durations, and which don't significantly reduce the maximum output power delivered by the circuits protected.